A process has heretofore been known for producing an alpha-olefin polymer, particularly an ethylene polymer or an ethylene/alpha-olefin copolymer by polymerizing ethylene or copolymerizing ethylene with an alpha-olefin in the presence of a titanium-type catalyst comprising a titanium compound and an organoaluminum compound or a vanadium-type catalyst comprising a vanadium compound and an organoaluminum compound. Generally, ethylene/alpha-olefin copolymers obtained with the titanium-type catalyst have a broad molecular weight distribution and a broad composition distribution. Ethylene/alpha-olefin copolymers obtained with the vanadium-type catalyst have a narrower molecular weight distribution and composition distribution than those obtained with the titanium-type catalyst. In some applications, the ethylene/alpha-olefin copolymers are sometimes desired to have a narrower molecular weight distribution and a narrower composition distribution.
Catalysts comprising zirconium compounds and aluminoxanes were recently proposed as a new Ziegler-type olefin polymerization catalyst.
Japanese Laid-Open Patent Publication No. 19309/,1983 (corresponding to U.S. Pat. No. 4,542,199) discloses a process which comprises polymerizing ethylene with at least one alpha-olefin having 3 to 12 carbon atoms at a temperature of -50.degree. to 200.degree. C. in the presence of a catalyst comprising a transition metal-containing compound represented by the following formula EQU (cyclopentadienyl).sub.2 MeR'Hal
wherein R' represents cyclopentadienyl, C.sub.1 -C.sub.6 alkyl or halogen, Me represents a transition metal, and Hal represents halogen, PA1 wherein R.sup.2 represents methyl or ethyl, and n is a number of 4 to 20, PA1 wherein R.sup.1 represents a cycloalkadienyl group, R.sup.2, R.sup.3 and R.sup.4 are identical or different and each represents a cycloalkadienyl group, an aryl group, an alkyl group, an aralkyl group, a halogen atom, or a hydrogen atom, Me represents zirconium, titanium or hafnium, k is a number of 1 to 4, l, m and n are positive integers, and k+l+m+n=4. PA1 preferably E.sub.x .ltoreq.2.2-2.times.P.sub.EW, PA1 more preferably E.sub.x .ltoreq.1.35-P.sub.EW, PA1 especially preferably E.sub.x .ltoreq.0.9-0.6.times.P.sub.EW. PA1 1.0+0.3xP.sub.E .ltoreq.B.ltoreq.1/(1-P.sub.E) PA1 more preferably PA1 1.0+0.4xP.sub.E .ltoreq.B.ltoreq.1/(1-P.sub.E), PA1 especially preferably PA1 1.0+0.5xP.sub.E .ltoreq.B.ltoreq.1/(1-P.sub.E), PA1 1.3-0.3xP.sub.E .ltoreq.B.ltoreq.1/P.sub.E, PA1 more preferably PA1 1.4-0.4xP.sub.E .ltoreq.B.ltoreq.1/P.sub.E, PA1 especially preferably PA1 1.5-0.5xP.sub.E .ltoreq.B.ltoreq.1/P.sub.E. PA1 Temperature: 120.degree. C. PA1 Frequency: 25.05 MHz PA1 Spectral width: 1500 Hz PA1 Filter width: 1500 Hz PA1 Pulse repeating time: 4.2 sec PA1 Pulse width: 7 microseconds PA1 Integration cycles: 2000 to 5000
and a linear aluminoxane represented by the following formula EQU Al.sub.2 OR.sub.4.sup.2 (Al(R.sup.2)--O).sub.n
or a cyclic aluminoxane represented by the following formula ##STR1## wherein R.sup.2 and n are as defined. This patent document states that to adjust the density of the resulting ethylene copolymer, ethylene should be polymerized in the presence of a small amount (up to 10% by weight) of a slightly long-chain alpha-olefin or a mixture of such alpha-olefins.
Japanese Laid-Open Patent Publication No. 95292/1984 (corresponding to U.S. Pat. No. 4,544,762) describes an invention relating to a process for producing a linear aluminoxane represented by the following formula ##STR2## wherein n is 2 to 40 and R.sup.3 represents a C.sub.1 -C.sub.6 alkyl group,
and a cyclic aluminoxane represented by the following formula ##STR3## wherein n and R.sup.3 are as defined. This patent document states that when ethylene is polymerized in the presence of a mixture of methylaluminoxane produced by the above process and a titanium or zirconium bis(cyclopentadienyl) compound, polyethylene can be obtained in an amount of at least 25 million grams per gram of the transition metal per hour.
Japanese Laid-Open Patent Publication No. 35005/1985 (corresponding to U.S. Pat. No. 4,536,484) discloses a process for producing an olefin polymerization catalyst, which comprises reacting an aluminoxane compound represented by the following formula ##STR4## wherein R.sup.4 is C.sub.1 -C.sub.10 alkyl, and R.sup.o together forms a bivalent --O-- radical or represents R.sup.4,
with a magnesium compound, chlorinating the reaction product, and treating the chlorinated product with a compound of Ti, V, Zr or Cr. This patent document states that the resulting catalyst is especially suitable for copolymerization of a mixture of ethylene with a C.sub.3 -C.sub.12 alpha-olefin.
Japanese Laid-Open Patent Publication No. 35006/1985 (corresponding to European Patent Application No. 128046A) discloses a combination of (a) a mono-, di- or tri-cyclopentadienyl of two or more different transition metals or a derivative thereof and (b) an alumoxane (aluminoxane). Example 1 of this patent document discloses that polyethylene having a number average molecular weight of 15,300 and a weight average molecular weight of 36,400 and containing 3.4% of propylene was obtained by polymerizing ethylene and propylene using bis(pentamethylcyclopentadienyl) zirconium dimethyl and alumoxane. In Example 2, a blend of polyethylene and an ethylene/propylene copolymer having a number average molecular weight of 2,000 and a weight average molecular weight of 8,300 and containing 7.1 mole % of propylene composed of a toluene-soluble portion having a number average molecular weight of 2,200 and a weight average molecular weight of 11,900 and containing 30 mole % of propylene and a toluene-insoluble portion having a number average molecular weight of 3,000 and a weight average molecular weight of 7,400 and containing 4.8 mole % of propylene was obtained by polymerizing ethylene and propylene using bis(pentamethylcyclopentadienyl) zirconium dichloride, bis(methylcyclopentadienyl) zirconium dichloride and alumoxane as a catalyst. Likewise, Example 3 describes a blend of LLDPE and an ethylene/propylene copolymer composed of a soluble portion having a molecular weight distribution (Mw/Mn) of 4.57 and containing 20.6 mole % of propylene and an insoluble portion having a molecular weight distribution of 3.04 and containing 2.9 mole % of propylene.
Japanese Laid-Open Patent Publication No. 35007/1985 (corresponding to European Patent Application No. 129368A) discloses a process which comprises polymerizing ethylene alone or with an alpha-olefin having at least 3 carbon atoms in the presence of a catalyst system comprising a metallocene and a cyclic alumoxane represented by the following formula ##STR5## wherein R.sup.5 represents an alkyl group having 1 to 5 carbon atoms, and n is an integer of 1 to about 20,
or a linear alumoxane represented by the following formula ##STR6## wherein R.sup.5 and n are as defined above. The polymer obtained by this process, according to the description of this patent document, has a weight average molecular weight of about 500 to about 1,400,000 and a molecular weight distribution of 1.5 to 4.0.
Japanese Laid-Open Patent Publication No. 35008/1985 (corresponding to U.S. Pat. No. 4,530,914) describes that polyethylene or an ethylene/C.sub.3 -C.sub.10 alpha-olefin copolymer having a broad molecular weight distribution is produced by using a catalyst system comprising at least two metallocenes and alumoxane, and that this copolymer has a molecular weight distribution of 2 to 50.
Japanese Laid-Open Patent Publication No. 35009/1985 (corresponding to U.S. Pat. No. 4,540,753) discloses a process for producing a copolymer of ethylene and an alpha-olefin having a molecular weight distribution of as small as less than 2 by using a catalyst system comprising a vanadium compound and an organoaluminum compound.
It is an object of this invention to provide an industrially advantageous process which can produce an alpha-olefin polymer having a high molecular weight using a novel catalyst comprising a compound of a transition metal of Group IVB of the periodic table and an aluminoxane.
Another object of this invention is to provide a process for producing a homopolymer and a copolymer of an alpha-olefin having a narrow molecular weight distribution and/or a high molecular weight.
Still another object of this invention is to provide a process which can produce a copolymer of an alpha-olefin having a narrow composition distribution and a high molecular weight.
Further objects and advantages of this invention will become apparent from the following description.